Airplane instrument for furnishing a bias signal to offset the effects of forward components of gusts during landing approach



L'; M. GREENE June 27. 1967 3,327,972 AIRPLANE INSTRUMENT FOR FURNISHINGA BIAS SIGNAL T0 OFFSET THE EFFECTS OF FORWARD COMPONENTS OF GUSTSDURING LANDING APPROACH Filed March 4, 1965 5 Sheets-Sheet l mMFDmEOO0732200 Qmmam Kurt E24 @5223 NW wfi 3 E @S g m: a 9: w: A N3 Q P a i Q?mm Lw l m mbf wm rvh Q \N\ Edi @E a a &

INVENTOR.

LEONARD M. GREENE BY gg ficw 62;,

ATTORNEYS June 27. 1967 L EN 3,327,972

AIRPLANE INSTRUMENT FOR FURNISHING A BIAS SIGNAL To OFFSET THE EFFECTSOF FORWARD COMPONENTS OF GUSTS DURING LANDING APPROACH Filed March 4,1965 3 Sheets-Sheet 2 LEONARD M. GREENE ATTORNEYS NG LANDING APPROACH 5SHeets-Sheet 3 90 vm N June 27. 1967 AIRPLANE INSTRUMENT FOR FURNISHINGA BIAS SIGNAL TO OFFSET THE EFFECTS OF FORWARD COMPONENTS OF GUSTS DURIFiled March 4, 1965 GEE E2, 02-2215 v United States Patent 3,327,972AIRPLANE INSTRUMENT FOR FURNISHING A BIAS SIGNAL TO OFFSET THE EFFECTSOF FORWARD COMPONENTS OF GUSTS DURING LANDING APPROACH Leonard M.Greene, Chappaqua, N.Y. 10514 Filed Mar. 4, 1965, Ser. No. 437,055 25Claims. (Cl. 24477) This invention relates to an airplane instrument forfurnishing a bias signal to offset the effects of forward components ofgusts during landing approach.

Shifting winds or gusts cause fluctuations in the instantaneous airspeed of an airplane. For example, a head gust, Le, a suddenrearwardshort term increase in the speed of a local air mass relative toan airplane, increases the instantaneous air speed of the airplane. Justprior to the incidence of the gust the airplane in smooth air has acertain instantaneous forward air speed relative to the moving orstationary local air mass, to which upon the occurrence of the head gustis added the sudden rearward increase in the speed of the local airmass. Since such a gust enhances air speed, it is referred to as apositive" gust. A tail gust, on the other hand, constitutes a suddenforward short term increase in speed of the local air mass and hencedecreases the air speed of the airplane. Accordingly, it is referred toas a negative gust. It is the fore-and-aft (relative to an airplane)component of any gust that has such a positive or negative effect on theair speed of the airplane.

During a landing approach when an airplane is flying at an air speed orlift value which is low, in order to obtain a desirable low landingspeed, but yet is safely above the minimum air speed or lift valuenecessary to maintain the airplane in flight, or correspondingly, whenthe airplane is flying at a desirable angle of attack which is high forthe purpose of enabling operation at a desirable low landing speed, butyet is safely below the maximum angle of attack at which stall wouldoccur, a negative gust, which is to say, a gust having a negativecomponent, creates a problem in that it momentarily, for the duration ofthe gust, tends to give rise to too low'an air speed or lift value, ortoo high an angle of attack. Such tendency must be overcome by theapplication of a higher level of power, i.e., an advance in throttlesetting, so as to keep the air speed, lift valuev and angle of attack atsafe desirable levels at all times throughout a landing approach. Aconsequent disadvantage is that the amount of power increase required tonullify the fluctuation in air speed or lift value or angle of attack isexcessive and the ensuing surging of the engines is both uncomfortableand unsafe.

At present, an airplane pilot, in order to avoid engine surging, whicharises from throttle compensation for gusts on an individual gust basis,makes it a practice to counter gusts in an empirical quasi-arbitrarymanner. Under conditions wherein a train of gusts of random intensity,duration, direction and occurrence is encountered, a pilot will fly theairplane at an average speed that is somewhat faster than minimum safeair speed, so that negative gust fluctuations will not bring about adangerous air speed, lift value or angle of attack. In this manner theamount of power manipulation that is required is considerably reduced.Nevertheless, this empirical solution of an arbitrarily selectedincreased speed is not satisfactory because it relegates the degree ofoverspeed to the pilots discretion which may not always be accuratelyexercised and which will vary from time to time and from individual toindividual. i

It is .a principal object of my present invention to provide aninstrument which willfurnish a bias signal to offset the effects ofnegative gusts during landing approach which instrument will operate ina predetermined precise 3,327,972 Patented June 27, 1967 and accuratefashion so as not to leave to pilot discretion the power regulation thatis to be exercised to offset gusting.

It is another object of my invention to provide an instrument of thecharacter described which is reliable, efficient and safe in operation,so that the security of airplanes and airplane passengers may properlybe entrusted thereto.

It is another object of my invention to provide an airplane instrumentof the character described which will respond rapidly to the firstnegative gust, so that the air speed of the airplane can be quicklyincreased to avoid the initial unsafe condition.

It is another object of my invention to provide an airplane instrumentof the character described wherein the increased speed to compensate fornegative gusting is maintained smoothly so long as negative gustingprevails, that is to say, an instrument in which the director, i.e.,command, signal to offset negative gusting is maintained for a period oftime sufliciently prolonged to offset a succession of negative gustswhereby intermittent engine surging is avoided.

It is another object of my invention to provide an airplane instrumentof the character described in which once negative gusting has occurredand the air speed of the airplane has been increased to offset theeffects thereof, a clamping level is established to which subsequentnegative gusts are compared and wherein further increases in air speedwill be commanded only if the negative gusting exceeds the clampinglevel thus tending to minimize the effect of negative gusts following aninitial negative gust.

It is another object of my invention to provide an airplane instrumentof the character described in which the aforesaid clamping level isvariable as a function of the average actual air speed or lift value orangle of attack of the airplane which has resulted from obeying thecommand signal that initially was created by the negative gustingcondition.

It is another object of my invention to provide an airplane instrumentof the character described in which the increased air speed commandsignal is slowly reduced after negative gusting ceases, so that theairplane will be restored to normal equilibrium condition in the flightmaneuver that is being carried out, to wit, landing approach.

It is another object of my invention to provide an airplane instrumentof the character described which does not respond to positive gusts andtherefore will not call for a reduction in commanded speed (director:air speed signal) because of a positive gust, although it will permit agradual reduction in the increased air speed command signal after thenegative gusting condition has passed.

It is another object of my invention to provide an airplane instrumentof the character described in whichthe variable clamping levelsubstantially eliminates fluctuating speed command signals once a safeaverage air speed or lift value or angle of attack has been reached.

It is another object of my invention to provide an airplane instrumentof the character described which constitutes relatively few and simplecomponents and which is light in weight and reliable in .operation.

Other objects of my invention in part will be obvious and in part willbe pointed out hereinafter.

My invention accordingly consists in the features of construction,combinations of elements and arrangements of parts which will beexemplified in the instruments hereinafter described and of which thescope of application will be indicated in the appended claims.

In the accompanying drawings in which are shown various possibleembodiments of my invention,

FIG. 1 is a schematic circuit diagram of an airplane instrumentembodying my present invention for furnishing an air speed directorsignal which, inter alia, is a func- -13 tion of negative gustingconditions, including both an initial negative gust and the integratedeffect of a series of negative gusts;

FIG. 2 is a diagram illustrating a modification of FIG. 1 in which theprevailing average lift condition of the airplane is derived from an airspeed sensor instead of from a lift vane as in FIG. 1;

FIG. 3 is a diagram illustrating an angle of attack sensor which can beused instead of the lift vane shown in FIG. 1; and

FIG. 4 is a diagram illustrating another modification of FIG. 1 in whichthe rate of change of air speed of the airplane is derived bycomputation from the outputs of sensors that are responsive to variablesother than air speed and of which the air speed is a function,specifically vertical acceleration and lift value, rather than, as inFIG. 1, from the output of a sensor that is directly responsive to airspeed.

In general, I carry out my invention by providing a first means theoutput of which is a function of the rate of change of air speed and byproviding a second means the output from which is a function of theprevailing average lift condition of the airplane.

The first means may directly sense rate of change of air speed, or, moreconventionally, it may directly sense air speed and by computation, towit, differentiation, as with a series capacitor if the air speed isrepresented by voltage, derive the rate of change of air speed.Alternatively the first means may provide a rate of change of air speedsignal that is derived by computation from the outputs of plural sensorsthat are individually responsive to variables other than air speed butof which air speed is a function. For example, air speed is a functionof vertical acceleration and angle of attack; it also is a function ofvertical acceleration and lift value. Hence, if sensors are already inplace that are responsive to vertical acceleration and to angle ofattack or lift value, as they are on many airplanes, and if no sensor isin place that is responsive to air speed, rate of change of air speedcan be obtained by combining the outputs of such plural sensors and bydifferentiation, the combining being in a proper proportion to yield asignal that is the equivalent of a signal that would be obtained bydirectly measuring rate of change of air speed (fluctuation in airspeed) or by measuring change in air speed and differentiating the same.

The second means having an output that is a function of the prevailingaverage lift condition of the airplane may sense any property which isrelated to lift condition and provides an output that is an average(slowly responsive) function thereof. For example, the property sensedby the second means may be the lift value as determined by the positionof a lift vane or the angle of attack as determined by the position ofan angle of attack vane or the air speed of the airplane as determinedby an air speed measuring device or as determined by computation fromvariables of which air speed is a function. The output from the secondmeans must be nulled with respect to a predetermined desirable value ofair speed or a predetermined desirable value of lift or a predetermineddesirable value of angle of attack, so that it is a function of thedeviation from null. The output from said second means is damped, sothat such output by having its speed of response slowed down indicatesan average condition extending over at least one second and preferably afew seconds, for example, five seconds, whereby this second output doesnot suddenly vary from moment to moment. Phrased differently, the outputfrom the second means has a delayed response, the period of delay, i.e.,the averaging period, preferably being greater than the median gustfrequency, which is about one second, so that the output from the secondmeans is far less sensitive to gusts than is the output from the firstmeans.

The outputs of the first and second means are compared in a suitablesumming means, i.e., a computer, and

the output from such summing means is fed to an integrator (a storagemeans) through a polarizing device which restricts the signals fed tothe integrator to signals that indicate the presence of a negative gustcondition for which a speed correction should be made. In other words,the signals reaching the integrator are restricted to signals thatindicate a reduced air speed condition, this being equivalent to anincreased angle of attack condition and to a stall approachingcondition. The summing means is characterized by its ability to quicklypass a negative gusting signal and the integrator is characterized byits ability to react quickly to a negative gusting signal issuing fromthe summing means, so that the integrator will rapidly furnish an outputsignal upon the occurrence of an initial negative gust. The integratorfurther is so constructed that it will tend to maintain for a prolongedspan the condition it assumes upon receipt of the initial negativegusting signal, whereby it will over a period of time, such forinstance, as a minute or two, supply a slowly decaying output signalthat commands an increased air speed, whereby the engines will not beimmediately reduced in power upon the termination of a negative gust andthen quickly thereafter again increased in power upon the occurrence ofa subsequent negative gust, but rather will continue to operate smoothlyat a higher, slowly decreasing throttle (power) setting so as to createan increased air speed for a period of time long enough to avoid surgingand so that the increased air speed will still prevail when freshnegative gusts are subsequently encountered after not too long a periodof time, e.g., within a minute.

It will be observed that the airplane instrument actually forms part ofa loop which includes the airplane; that is to say, upon the occurrenceof a negative gust or series of such gusts the integrator will supply anair speed director signal which, when followed either automatically orby a pilot, will control, i.e., bring about an increase in, average airspeed. This raised average air speed is reflected by an increase in theoutput of the second means above the predetermined desirable nullcondition. Since the summing means compares the outputs of the first andsecond means, subsequent negative gusts are measured against theincreased average air speed or increased average lift value or loweredaverage angle of attack. So long as the negative gusts do not exceed theover-null clamping signal of the second means, the integrator willcontinue to furnish an approximately even signal, so that an averagesafe increased air speed or increased safe lift value or decreased safeangle of attack will be reached in equilibrium condition of the loop.Hence, subsequent negative gusts, once the average air speed has beenincreased to reach the command signal, will not tend to cause an unduefurther increase in the command signal.

The integrator means furnishes a bias output signal which may be used inany suitable manner, as for example, by feeding the same into a speedcommand computer that operates a utilization mechanism such as an airspeed command meter or an automatic throttle control, or by adding thesignal to the output of a speed command computer in a second summingmeans the output from which is led to a utilization mechanism.

Referring now in detail to the drawings, and, more particularly to FIG.1, the reference numeral 10 denotes an airplane instrument constructedin accordance with my invention and illustrating an embodiment thereofwherein the sensors are of a mechanical nature and the variations inpositions thereof are transduced to electrical quantities and in whichsuitable circuit means are operated by said electrical quantities toeffect the various functions of the instrument.

Said instrument includes an air speed sensor 12 and a lift sensor 14each having an affiliated transducer.

The air speed sensor 12 is conventionl, the same being of a dymamicpressure type. It includes a case 1-6 provided with a single opening 18that connects the interior of the case, as through a tube 20, to theprevailing static pressure, that is to say, the ambient static airpressure of the local air mass in which the airplane is situated. Alsolocated within the case is a corrugated bellows 22 which expands orcontracts as a function of the difference in pressures between theinterior of the bellows and the interior of the case 16. A tube 24extends from the bellows through a wall of the case, to which it istightly sealed, to a forwardly facing Pitot head external to theairplane and sufficiently far from the airplane wing, propellers,engines and fuselage structure to be materially unaffected by turbulencecreated by the airplane. Thereby, the air pressure within the bellows isthe total Pitot pressure, including the static pressure which is afunction of altitude and air conditions and the dynamics pressure thatis a function of indicated air speed. Hence, the wall 26 of the bellowswill experience movement which is a function of Pitot pressure lessstatic pressure and hence of dynamic air pressure, and, therefore, afunction of prevailing indicated air speed.

The lift sensor 14 constitutes a vane 28 which extends through a slot 30in a mounting plate 32 that is secured over an opening 34 in the skin ofthe wing adjacent the nose thereof. Located behind the skin of the wingis a transversely (spanwise) extending pivot 36 for the vane which is sopositioned that it is behind the center of pressure of the vane.Suitable means, such, for instance, as a pair of opposed springs 38, 40,are provided to bias the vane to an equilibrium position between stops.Said vane is so located at the nose of the airplane that it is subjectedto variation in pressure caused by shifting of the stagna tion pointover the nose of the wing. The particular location of the vane on thenose and the strength of the springs which bias the vane to a neutralposition are not critical for proper performance of the instrument 10.The angular position assumed by said vane is a measure of the prevailingvalue of lift.

The output of the air speed sensor 12 as manifested by physical movementof the wall 26 is transduced into an electrical quantity, specificallyvoltage, by a potentiometer 42 consisting of a fixed winding 44 and amovable wiper 46. The wiper is driven by the wall 26. The ends of thepotentiometer winding 44 are connected to a suitable D.C. source, forexample, a battery 48. Hence, the voltage output E across the lead 50(connected to the wiper 46) and the lead 52 (connected to a terminal ofthe battery 48) will be a function of air speed. This output voltage isconverted to rate of change of air speed voltage ERCAS by adifferentiating means, to wit, a series capacitor 54, which is insertedin the lead 50. Said voltage is fed to an input coil 56.

The polarization of the battery and the direction of movement of thewiper 46 responsive to change in prevailing air speed is such that upona decrease in air speed the terminal of the coil 56 which is connectedto the capacitor 54 will be negative with respect to the other terminalof the coil. These respective polarities are indicated by the plus andminus signs at opposite ends of the coil 56. The particular polaritiesselected are purely arbitrary. However, they supply reference polaritiesfor other polarities later to be described. It will be apparent that ata constant air speed the voltage ERCAS will be zero and that a voltageis developed across the coil 56 only by virtue of a change of thesetting of the potentiometer wiper 46, this voltage decaying by passageof a current i through said coil.

Said input coil 56 is one of plural input coils of a suitable firstsumming amplifier 58 such as a reset magnetic amplifier. A typicalamplifier of this type is the Ferrac magnetic amplifier, manufactured byAirpax Electronics, Seminole Division, Fort Lauderdale, Fla. This typeof amplifier includes a plurality of control inputs of which the inputcoil 56 is one, and a polar output the terminals 60, 62 of which areshown. Only two input control coils 56, 64 have been illustratedinasmuch as these are the only ones necessary for use in my instrument10.

The output of the lift sensor 14 as manifested by physical movement ofthe vane 28 is transduced into an electrical quantity, specificallyvoltage, by a resistance bridge 66. Two legs of the bridge constitute aresistance voltage divider in the form of a potentiometer having a fixedwinding 68 with a movable wiper 70. The ends of said potentiometer whichare the input terminals of the bridge are connected to a suitable DCsource, such as a battery 72. The other two legs of the bridgeconstitute a second potentiometer having a fixed resistance winding 74and a wiper 7-6. The ends of the two potentiometers are connected to oneanother, so that said potentiometers are in parallel and both areconnected across the battery 72. The wiper 76 is driven by the vane 28.With increased lift which is a result of increased air speed, the vane28, as shown in FIG. 1, will turn in a counterclockwise direction, sothat the potential on the bridge output lead 78 Will become morenegative (or less positive) with respect to the potential on the bridgeoutput lead 80.

It will be apparent that the position of the wiper 70 furnishes a nullpoint for the output on the leads 78, 80, that is to say, with apredetermined position of the wiper 70 in a certain position of thewiper 76 the output on the leads 78, 80 will be zero. At any position ofthe lift vane 28 indicating a higher lift value than the null valuecorresponding to the position of the wiper 70, the lead 78 becomesnegative with respect to the lead 80 and vice versa. The wiper 70 is seteither manually or automatically for any preselected desirable liftvalue, for example, a lift value corresponding to an air speed which issafely above, by a desirable amount, the minimum air speed duringlanding approach. If the lift value increases, as with an increase inair speed, above the null value the lead 78 will go negative withrespect to the lead 80 by an amount which is a function of the value oflift in excess of the set null value represented by the set position ofthe wiper 70.

The output from the lift sensor 14 is delayed so that it is averaged andthereby is less sensitive to gusts than is the output from the airspeedsensor 12. This can be accomplished in various manners. Forinstance, the movement of the vane 28 may be hydraulically restrained.As shown herein, the voltage across the leads 78, 80 is damped foraveraging purposes by an RC filter circuit constituting a capacitor 82connected across said leads, an input resistor 84 in the lead 80 and anoutput resistor 86 in the lead '88. The leads 78, 88 are connected tothe input control coil 64 so that the voltage E (voltage that is afunction of the deviations of averaged lift value from a preselecteddesirable null lift value) across said leads induces a flow of current itherein and thus provides a second input to the summing amplifier 58which now will be seen to have an input ERCAS which is a function of therate of change of air speed, and another input E which is a function ofthe deviation of average lift value from a null preselected desirablelift value. It will be noted that the currents i and i are summed in alike sense, that is, they are cumulative for increasing air speed,increasing lift value and decreasing angle of attack both of whichlatter parameters are associated with increasing air speed. Hence, thecurrents i and i will create opposing effects in the summing amplifier58 when the current i is representative of a negative rate of change ofair speed and the current i is representative of an increased averageair speed (lift value) over the null air speed (lift value).

The values of the capacitor 82, resistors 84 and 86 and inductance 64are so selected that they have an RC time constant of the several, e.g.,five, seconds so that the current i represents the average value of thelift over the null value for a period of several seconds.

The polarity signs indicated at opposite ends of the input coil 64 arethose for a position of the lift vane 28 corresponding to an averagelift value greater than the preselected desirable null lift value set bythe position of the wiper 70 and have been shown in this manner becausethis is the condition of the circuit shortly after the occurrence of anegative gust condition and which the circuit has been speciallydesigned to handle. 'It will be recalled that the polarity signs at theopposite ends of the coil 56 are representative of circuit values for anegative gust condition.

The output from the first summing amplifier 58 which output appears atthe terminals 60, 62 is fed through leads 90, 92 to a small ripplefilter in the form of a capacitor 94 connected across said leads and apair of resistors 96, 98. The ripple filter removes noise and highfrequency or hash voltage fluctuations from the output of the amplifier58 which are introduced by the standard 400 cycle frequency of its powersupply. The resistor 96 is connected in the input leader 90 and theresistor 98 is an output lead 100 from the ripple filter.

The values for the capacitor 94 and resistors 96, 98 are such as tojointly present a small impedance and a fast time constant. In order topermit a rapid flow of current through the summing amplifier 58 theinternal impedance of said amplifier is quite low, e.g., in the order of50 ohms.

The output leads 100, 92 from the ripple filter are connected toopposite terminals of an integrating (storage) capacitor 102 which isdesigned to be charged by the output from the first summing amplifier. Aresistor 104 is inserted in an output lead 106 from one terminal of thestorage capacitor 102, the value of said resistor and said capacitorbeing so chosen that their RC time constant is comparatively prolonged,for example, about one minute.

An essentially unidirectional conducting device such as a diode 108 isinterposed between the resistor 98 and a terminal of the storagecapacitor. Said diode is so oriented that the storage capacitor can becharged only by a voltage at the output of the summing amplifier 58which voltage is negative at the terminal 60 and positive at theterminal 62, suitable positive and negative symbols being provided inFIG. 1 to indicate this polarity condition.

By virtue of the foregoing arrangement the capacitor 102 will be chargedwhen, and only when, the output from the summing amplifier is negativeat the terminal 60 and positive at the terminal 62 and this conditionwill be found solely when there is a negative gust condition (Le, areduced air speed condition which is equivalent to an increased angle ofattack condition and to a stall approaching condition), so that theupper end of the coil 56 is negative with respect to the lower end, andthere either is no current i flowing in the coil 64 or if the polarityat the upper end of the coil is positive with respect to the lower enddue to an increase in average lift value over the null value, the valueof the voltage across the coil 64 has an effect which is less than theeffect of the value of the voltage across the coil 56. In other words,the storage capacitor 102 will charge only when the effect of a negativegust condition exceeds the effect of the average variation of theposition of the lift vane 28 from null position.

The leads 111 112 at the output of the integrating storage means whichconstitutes the storage capacitor 102 and the resistor 104 thus haveimpressed across them a bias voltage E that initially is a function ofthe first of a train of negative gusts and subsequently is a function ofthe stored memory of such initial negative gust or the degree to whichsubsequent negative gusts exceed the average increased lift condition Ewhich has been brought about in response to an air speed command signal.

Said potential E across the leads 110, 112 is employed to bias an airspeed command signal so as to call for increased air speed and for thispurpose is fed into a suitable utilization device, for example, saidpotential can be fed into an input control coil of a speed commandcomputer 114 having output terminals connected to an air speed command(director) meter 116. Such a speed command computer is shown, forexample, in my copending application Ser. No. 316,759, filed Oct. 16,1963, for Airplane Instrument for Furnishing a Deceleration ModifiedDirector Signal to Indicate or Control Corrective Action for OffsettingDecrease in Head Wind During Landing Approach, or in my Patent No.3,043,540, issued July 10, 1962, for Airplane Instruments.

In the circuit illustrated in FIG. 1, I have illustrated a modifiedvariation of the foregoing arrangement for utilizing the biasing signalE wherein the leads 110, 112 are connected to the input coil 118 of asecond summing amplifier 120 of the same type as the summing amplifier58. The summing amplifier has a second input coil 122 connected by leads124, 126 to the output terminals of the speed command computer 114. Thespeed command computer impresses a potential E on the input coil 122that is a function of a command air speed, that is to say, if theairplane is going too slow for a given set of parameters, the current iflowing through the coil 122 reflects this too slow condition. Suchparameters are exclusive of the correction for negative gusts. Thecurrent i flowing in the coil 118 is a function of the negative gustconditions. The currents i and L, are summed in a cumulative sense forincreasing air speed, so that if the potential appearing across theleads 124, 126 is indicative of a slow air speed condition and thepotential appearing across the leads 110, 112 likewise is indicative ofa slow air speed condition, the effects of the two coils 118, 122 willbe added.

The summing amplifier 120 has a polarized output coil with outputterminals 128, 130 connected to leads 132, 134 that run to a utilizationmechanism such for example as an automatic throttle control or the airspeed command meter 116. If the potential appearing across the leads124, 126 is such as to indicate that the airplane is flying at apreselected desirable air speed for all parameters during landingapproach except negative gusts and if the potential E appearing acrossthe gusts leads 110, 112 is such as to call for an increased air speed,the potential appearing across the leads 132, 134 will call for(command) an increased air speed in the automatic throttle or the meter116.

Alluding specifically to the meter 116, such a signal will swing theneedle of said meter to the slow side of the null central position. Whenthe pilot sees the meter in slow position he will increase his throttlesetting so as to speed up the airplane and this will bring the needleback to the null position by virtue of the increased output from thespeed command computer resulting from a faster signal issuing therefromdue to the increased lift input fed to said computer. This is the loopthat will increase lift value as will be evidenced by an increase in thevoltage E across the leads 78, 88-.

The operation of the instrument 10 will now be described.

When the airplane is flying at a predetermined desirable average liftvalue, which value is to be maintained during landing approach andcorresponds to a predetermined desirable landing approach average airspeed, i will be zero, or substantially so, inasmuch as the positionassumed by the vane 28 will correspond to the setting of the wiper 70'.If at this time there are no negative gusts the current i will be zero.The storage capacitor 102 will not be charged so that the current i willbe zero. The current i.,, if the speed command computer is a nulledinstrument, will also be zero and the meter 116- will have its needle innull position, or the automatic throttle will be at a setting whichmaintains the desired average air speed and average lift condition.

Now assume that a negative (tail wind) gust develops. i will not changeimmediately because of the time constant of the capacitor 82 andresistors 84, 86 which is several, e.g., about five seconds. However, iimmediately starts to fiow in the negative sense indicated by thepolarity signs associated with the coil 56. This direction of flowsignals a speed that is too slow, which is equivalent to directing apilot to increase speed. Said signal will be passed by the summingamplifier 58 without any modification from the coil 64 (because i stillis zero or is only slowly starting to flow) and will quickly charge upthe storage capacitor 102 inasmuch as the time constant of the capacitor94 and the resistors 96, 98 is very small. The potential from thecapacitor 102 will cause a flow of current i in the coil 118 in adirection to deflect the needle of the meter 116 to the slow side.

Such indication in the meter or any other utilization mechanism employedwill take place immediately upon the occurrence of the first negativegust. The pilot will advance the throttle setting to increase the airspeed of the airplane so as to counter the effect of this first negativegust.

The time constant of the storage capacitor 102 and associated resistor 4is quite lengthy, for example, about a minute. Hence, it takes thisperiod of time for its charge to be materially reduced and over thisperiod the gust bias influence on the command signal will indicate thatthe air speed is to be increased. Subsequent negative gusts tend to keepthe capacitor 102 charged.

As a result of the command to the pilot or to the automatic throttlecontrol to increase air speed, the setting of the vane 28 will bechanged to correspond to the increased average lift value. This willcause the current i to flow in the coil 64 in the direction indicated bythe polarity signals associated with that coil. Such a signal indicatesan increased average lift value or air speed and hence opposes thesignal caused by the flow of the current i in the direction indicated inthe circuit. If the effect of the signal i -exceeds that of the signal ino further charge will be impressed on the capacitor 102 due to thepresence of the diode 108 which only permits charging of the capacitor102 in a certain direction corresponding to a negative gust and does notpermit discharge thereof in the opposite direction from the input sideof such storage capacitor. Thus, the combination of the diode 108 andthe coil 64 acts as a clamping means to clamp the charging of thestorage capacitor 102 at a variable clamping level which is a functionof average lift condition, this being manifested in the circuit of FIG.1 as an average lift value sensed by the lift vane 28. Thereby anequilibrium condition will be reached as to average air speed, liftvalue and angle of attack which will be unaffected by further gustsunless the gusts are of a greater value than the initial gusts or unlessthe further gusts occur after the charge on the storage capacitor 102has decayed and the average lift condition of the airplane has beenreduced to match this decayed value.

After a train of negative gusts has ceased and, a few minutes later whenthe capacitor 102 has substantially fully discharged, the current i inthe second summing amplifier 122 will reduce to zero and the originaloperating parameters will be restored. In a continual uniform negativegusting air mass balance conditions will be reached when the current iis just enough greater (more negative) than the clamping current i tokeep the storage capacitor 102 charged to a level which will yield an icurrent suflicient to make the airplane fly enough faster to keep the icurrent at the required value to result in such a balance.

It thus will be appreciated that the instrument 10' operates in a mannersuch as to. provide several desirable features. The initial negativegust will immediately command a higher airplane air speed. By followingthe command an increased air speed is maintained as long as required,that is to say, as long as subsequent negative gusts-continue to occur.The air speed, however, upon following the command is slowly reduced asconditions permit without objectionable rapid fluctuating changes inspeed command. The variable clamping level substantially eliminatesfluctuating speed commands once a safe average lift condition has beenreached. The response of the bias signal appearing across the leads 112is essentially unidirectional so that the potential applied to the inputcoil 118 can only command an increase and not a decrease of air speed;thus the circuit does not respond to positive gusts and therefore noreduction in air speed will he commanded as a result of such biassignal.

As indicated previously, the rate of change of air speed as evinced bythe voltage E which is a function of the amplitude of negative gusts, isbalanced for clamping purposes against an average lift condition whichis evinced in FIG. 1 as a voltage E Lift condition can be sensed by theprevailing lift value of the airplane or, alternately, by equivalentparameters such for instance as the prevailing air speed or theprevailing angle of attack. In FIG. 2 I have shown an instrument 10"which is the same as the instrument 10 except that for the lift sensor14 I have substituted an air speed sensor. Inasmuch as an air speedsensor is necessary to derive a voltage that is a function of the rateof change of air speed, the same air speed sensor is employed for bothpurposes.

Specifically I provide an air speed sensor 136 having a physicalconstruction identical to that of the air speed sensor 12. It includes acase 138 the interior of which is connected to prevailing ambient staticpressure by -a tube 140. Within the interior of the case is a bellows142 the inside of which is connected by a tube 144 to a forwardly facingPitot head in a region where it is minimally influenced by disturbingfactors. The wall 146 of the bellows, the position of which varies as afunction of the difference of the pressures externally and internally ofthe bellows, drives two wipers 148, which are mutually electricallyinsulated. The remainder of the instrument 10 the same as the remainderof the instrument 10 already described in detail. Thus, the wiper 148rides on the potentiometer winding 74 and the wiper 150 rides on thepotentiometer winding 44. The other elements of the circuit for theinstrument 10' up to the bias signal leads 110, 112 have been shown inFIG. 2 and have had reference numerals applied thereto which are thesame as those for corresponding elements of FIG. 1. Since all the otherparts of the instrument 10 are the same as those of the instrument 10they have not been shown. It will be observed that the output from thepotentiometer 74 is damped to obtain a voltage E that is a function ofaverage air speed rather than of air speed as instantaneously affectedby gusts.

In FIG. 3 I have illustrated an instrument 10" which is identical to theinstrument 10 shown in FIG. 1 except for the means to drive the wiper76. Since all other parts of the instrument 10" are the same as those ofthe instrument 10, they have not been shown. The instrument 10 senseslift condition by measuring angle of attack. For this purpose I employan angle of attack sensor 152 constituting an angle of attack vane 154affixed to an arm 156 that turns about a lateral shaft 158, to wit, ashaft perpendicular to the line of flight A and horizontal when theairplane is in level flight. The arm 156 drives the wiper 76 of thepotentiometer having the fixed winding 74 and through a damping RC'network controls the current i flowing in the coil 64 as a function ofaverage angle of attack.

As I have mentioned earlier, it is not essential to the construction of"an instrument embodying my present invention that the value of the rateof change of air speed engendered by gusts be taken from a first meansthat includes a means which directly senses and, therefore, directlymeasures air speed and from it computes rate of change of air speed. Anequally satisfactory equivalent result will be obtained if the firstmeans is subdivided into sundry subordinate sensing means and acomputing means, the sensing means being capable of measuning valuesfrom which air speed can be computed and the computing means beingcapable of performing calculations, including differentiation, upon saidvalues so as to derive the rate of change of air speed. The thusobtained rate of change of air speed is the equivalent of the rate ofchange of air speed obtained by directly measuring air speed anddifferentiating such measurement, and can be substituted for it.

In FIG. 4 I have shown an instrument which is identical to theinstrument 10 except for an altered construction of the first means tosupply the current i which is a function of the gust engendered rate ofchange of air speed (gust engendered fluctuations in air speed). Sinceall other parts of the instrument 10" are the same as those of theinstrument 10 they have not been shown.

The instrument 10" measures gust engendered rate of change of air speedby measuring vertical acceleration and lift value and from the changesthereof, by differentiation and calculation, obtaining rate of change ofair speed.

Specifically, in FIG. 4 I have illustrated only so much of theinstrument 10" as supplies the current i to the input control coil 56 ofthe summing amplifier 58. The components and connections of the circuitthat supplies the current i to the input coil 64 are the same as in FIG.1 and, therefore, have been omitted. For the same reason the componentsand connections of the circuit leading from the output terminals 60, 62via leads 90, 92 have been omitted.

The instrument 10' includes two sensors in the first means, to wit, avertical, i.e., normal, accelerometer 160 and a lift sensor 162 both ofwhich are conventional.

The vertical accelerometer comprises a weight 164 constrained in guides(not shown) to permit movement only along the Z-axis of the airplanewhich is perpendicular to the plane containing the X-axis (thelongitudinal axis of the fuselage) and the Y-axi-s (the longitudinalaxis of the wing span). The Weight is supported by a counterbalancing,i.e., centering, compression spring 166. The weight 164 drives a movableWiper *168 of a potentiometer 170 having a fixed Winding 172 connectedacross a suitable DC source such as a battery 174. The voltage E between a lead 176 from the wiper 168 and a lead 17 8 from one end of thepotentiometer is a function of the vertical acceleration of theairplane. A differentiating capacitor 180 series inserted in the lead178 converts the voltage E to a voltage ERCVA which is a function of therate of change of vertical acceleration and which is present across thelead 176 and a lead 182 extending from the capacitor 180.

The lift sensor 162 is identical to the lift sensor 14. Alternatively itmay be identical to the angle of attack sensor 152. Said sensor 162comprises a vane 184 which extends through a slot 186 in a mountingplate 188 that is secured over an opening 190 in the skin of the wingadjacent the nose (leading edge) thereof. Located behind the skin of thewing is a transversely (spanwise) extending pivot 192 for the vane whichis so positioned that it is behind the center of pressure of the vane.Suitable means, such for instance, as a pair of opposed springs 194,196, are provided to bias the vane to an equilibrium position betweenstops. Said vane is so located at the nose of the airplane that it issubjected to variation in pressure caused by shifting of the stagnationpoint over the nose of the wing. The particular location of the vane onthe nose and the strength of the springs 194, 196 are not critical forproper performance of the instrument 10'. The angular position assumedby the vane 1 84 is a measure of the prevailing lift value. In the eventthe sensor 162 is constructed like the sensor 152 the angular positionof the vane will be a measure of the prevailing angle of attack.

The vane 184 drives a movable wiper 198 of a potentiometer 200 having afixed winding 202 connected across a suitable DC source such as abattery 204. A potentiometer 286 has its fixed winding 208 connected inshunt with the winding 202 so that the two potentiometers form aresistance bridge for which the battery 204 supplies the input. Thebridge output appears across the lead 210 running from the wiper 198 andthe lead 212 running from the wiper 214 for the potentiometer 206.

The position of the wiper 214 establishes a null point for the output onthe leads 210, 212, that is to say, with a predetermined position of thewiper 214 in a certain position of the wiper 198 the output on the leads210, 212 will be zero. At any position of the lift vane 184 indicating ahigher lift value than the null value corresponding to the position ofthe wiper 214, the lead 210 becomes negative with respect to the lead212 and vice versa. The wiper 214 is set either manually orautomatically for any preselected desirable lift value, for example, alift value corresponding to an air speed which is safely above, by adesirable amount, the minimum air speed during landing approach. It thelift value increases, as with an increase in air speed, above the nullvalue the lead 210 will go negative with respect to the lead 212 by anamount which is a function of the value of lift in excess of the setnull value represented by the set position of the wiper 214.

In order to establish a proper proportion between the outputs from thevertical accelerometer and the lift sensor at least one such output,e.g., the output from the lift sens-or 162, is arranged to be adjustablewith respect to the other output. For this purpose the leads 210, 212are connected to the ends of a fixed winding 215 of a potentiometer 216having a wiper 218. The voltage E appearing between the lead 220 runningfrom the wiper 218 and the lead 222 running from an end of thepotentiometer winding 215 is a function of the lift value of theairplane having an amplitude range that can be adjusted by varying theposition of the wiper 218. A differentiating capacitor 224 seriesinserted in the lead 222 converts the voltage E to a voltage E which isa function of the rate of change of lift value and which is presentacross the lead 220 and a lead 226 extending from the capacitor 224.

A computer 230 combines the outputs ERCVA and E and supplies its ownoutput voltage ERCAS that is a function of the rate of change of airspeed and that appears across the output leads 232, 234. Because thecomputer is only required to operate within a narrow range of outputswhich represent gust engendered variations to a predetermined landingapproach air speed, it can be of simple construction and may, as shown,he in the form of a summing bridge. Three legs of the bridge constituteresistances 236, 238 and 240 while the fourth leg constitutes the inputcontrol coil 56 of the summing amplifier 58. The voltage ERCVA isapplied across the junction 242 between the resistor 236 and the coil 56and the junction 244 between the resistors 238 and 240. The voltage E isapplied across the junction 246 between the resistors 236 and 238 andthe junction 248 between the resistor 240 and the coil 56.

It will be apparent from the foregoing that the outputs from thevertical accelerometer and the lift vane (or angle of attack vane) areseparately fed to different capacitors 180, 224. The capacitors passfluctuating (rate of change) currents to the summing bridge 230 which,in turn, feeds the current i into the input coil 56 of the summingamplifier 58.

The output adjustment potentiometer 216 has its wiper 218 set so thatthe fluctuations of i will be responsive to fluctuations of the airspeed. This may be accomplished by adjusting the position of the wiper218 so that signal changes from the vertical accelerometer aresubstantially cancelled by signal changes from the lift sensor at anessentially fixed air speed. Thereby any unbalancing causing a flow ofcurrent i will be a measure of gust engendered fluctuations in airspeed.

The sources of voltage are so polarized and the directions of movementsof the wipers 168, 198 responsive to change in prevailing air speed aresuch that upon a decrease in air speed the terminal of the coil 56 whichis connected to the junction 242 will be negative with respect 13 to theother terminal of said coil as indicated .in FIG. 4. At a constant airspeed the voltage ERCAS will be zero.

Although I have shown the outputs E and E from the verticalaccelerometer and lift sensor as being differentiated to yield voltagesERCVA and E before being combined to calculate and furnish the voltage Eit is to be understood that said outputs E and E can also be combinedwithout differentiations in a computer such as the computer 230 tocalculate and yield at the computer output a voltage E which is afunction of the air speed of the airplane and that this later voltage Ecan be differentiated to yield the voltage ERCAS that is fed to theinput coil 56. Such an alternative circuit arrangement for the firstmeans is the same as that shown in full lines in FIG. 4 except that thecapacitors 180 and 224 are omitted, and, instead, a differentiatingcapacitor 250 (shown in dotted'lines in FIG. 4) is .series inserted inthe lead between the junction 242 and the upper terminal of the input.

The operation of the instruments 10" and 10" illustrated in FIGS. 2, 3and 4 are identical to the opertion of the instrument 10 alreadydescribed in detail.

It thus will be seen that I have provided devices which achieve theseveral objects-of my invention and which are well adapted to meet theconditions of practical use.

As various possible embodiments might be made of the above invention,and as various changes might be made in the embodiments set forth, it isto be understood that all matter herein described or shown in theaccompanying drawings is to be interpreted as illustrative and not in alimiting sense.

Having thus described my invention, I claim as new and desire to secureby Letters Patent:

1. For use in an airplane instrument that controls the air speed of anairplane during landing approach, means to furnish a bias signal tocommand an increase in air speed to offset the effect of negative gustson air speed during landing approach, said means comprising a firstmeans having an output which is responsive to gust en genderedfluctuations in air speed, a second means having an output which is afunction of the difference between the prevailing average lift conditionof the airplane and a predetermined desirable null lift condition, theoutputs of the first and second means being in the same negative sensefor decreasing air speed, a third means for comparing the outputs of thefirst and sec-0nd means and furnishing a difference output, and anintegrator responsive to and for storing substantially only a negativeoutput of the third means as said bias signal, such negative outputcorresponding to an approach to stall.

2. A bias signal furnishing means as set forth in claim 1 wherein thesecond means has an output which is a function of the difference betweenthe prevailing average air speed of the airplane and a predetermineddesirable null air speed.

3. A bias signal furnishing means as set forth in claim 1 wherein thesecond means has an output which is a function of the difference betweenthe prevailing average angle of attack of the airplane and apredetermined desirable null angle of attack.

4. A bias signal furnishing means as set forth in claim 1 wherein thefirst means includes a sensor that is directly responsive to air speed.

5. A bias signal furnishing means as set forth in claim 4 wherein thefirst means also includes means differentiating the signal from the airspeed sensor.

6. A bias signal furnishing means as set forth in claim 1 wherein thefirst means includes a vertical accelerometer, a lift sensor and acomputer to which are fed the signals from the vertical accelerometerand the lift sensor to calculate air speed therefrom.

7. A bias signal furnishing means as set forth in claim 6 wherein thefirst means also includes a differentiating means.

8. A bias signal furnishing means as set forth in claim 6 wherein thefirst means also includes means to differentiate the signals from thevertical accelerometer and the lift sensor before they are fed to thecomputer.

9. A bias signal furnishing means as set forth in claim 6 wherein thefirst means also includes means to differentiate the output from thecomputer.

10. A bias signal furnishing means as set forth in claim 1 wherein thefirst means includes a vertical accelerometer, a null lift vane, and acomputer to which are fed the signals from the vertical accelerometerand the null lift vane to calculate air speedtherefrom.

11. A bias signal'furnishing means as set forth in claim 1 wherein thefirst and second means include physical sensors and electric transducersto convert changes in condition of the physical sensors into variableelectrical characteristics and wherein the third means and theintegrator are electrical.

12. A bias signal furnishing means as set forth in claim 11 wherein allthe outputs are voltages.

13. A bias signal furnishing means as set forth in claim 11 wherein aunidirectional conducting device is interposed between the third meansand the integrator and is oriented to conduct the output from the thirdmeans to the'integrator only when the output from the first meansresponsiveto a negative gust condition exceeds the output from thesecond means responsive to an increased average lift condition.

14. A bias signal furnishing means as set forth in claim 13 wherein theintegrator has a prolonged time constant.

15. A bias signal furnishing means as set forth in claim 14 wherein thetime constant of the integrator is about one minute.

16. A bias signal furnishing means as set forth in claim 14 wherein thesecond means has a time constant of a few seconds.

17. A bias signal furnishing means as set forth in claim 13 wherein aspeed command computer also is provided and wherein the bias signal isconnected to an input of the speed command computer.

18. A bias signal furnishing means as set forth in claim 13 whereinthere also are provided a speed command computer with an output, asumming means with plural inputs and an output, and a utilizationmechanism with an input, the output of said speed command computer andthe output of the integrator being connected to the inputs of thesumming means, and the out-put of the summing means being connected tothe input of the utilization mechanism.

19. A bias signal furnishing means as set forth in claim 13 wherein thethird means is a summing means having plural inputs, one connected tothe output of the first means and the other to the output of the secondmeans, the inputs of the summing means being cumulative.

20. A bias signal furnishing means as set forth in claim 13 wherein theintegrator includes a capacitor.

21. A bias signal furnishing means as set forth in claim 13 wherein theintegrator is an RC network with a prolonged time constant.

22. A bias signal furnishing means as set forth in claim 13 wherein thefirst means includes a sensor providing a voltage responsive to airspeed and a series connected capacitor to differentiate said voltage toa voltage which is a function of the rate of change of air speed.

23. For use in an airplane instrument that controls the air speed of anairplane during landing approach, means to furnish a bias signal tocommand an increase in air speed to offset the effect of negative gustson air speed during landing approach, said means comprising a firstmeans having an output which is responsive to gust engenderedfiuctuations in air speed, a second means 'having an output which is afunction of the difference between the prevailing average lift conditionof the airplane and a predetermined desirable null lift condition, theoutputs of the first and second means being in the same sense forincreasing air speed, a third means for 15 comparing the outputs of thefirst and second means and furnishing a ditference output, and anintegrator responsive to and for storing as said bias signalsubstantially only an output of the third means correspond ing to areduced air speed condition.

24. For use in an airplane instrument that controls the air speed of anairplane during landing approach, means to furnish a bias signal tocommand an increase in air speed during landing approach, said meanscomprising a first means having an output which is responsive to gustengendered fluctuations in air speed, a second means having an outputwhich is a function of the difference between the prevailing averagelift condition of the airplane and a predetermined desirable null liftcondition, the outputs of the first and second means being in the samesense for increasing air speed, a third means for comparing the outputsof the first and second means and furnishing a diiference output, and anintegrator responsive to and for storing as said bias signalsubstantially'only an output of the third means corresponding to anincreased angle of attack condition.

25. For use in an airplane instrument that controls the air speed of anairplane during landing approach, means to furnish a bias signal tocommand an increase in air speed to offset the eifect of negative gustson air speed during landing approach, said means comprising a firstmeans having an output which is responsive to gust engenderedfluctuations in air speed, a second means having an output which is afunction of the difference between the prevailing average lift conditionof the airplane and a predetermined desirable null lift condition, theoutputs of the first and second means being in the same sense forincreasing air speed, a third means for comparing the outputs of thefirst and second means and furnishing a difference output, and anintegrator responsive to and for storing as said bias signalsubstantially only an output of the third means corresponding to a stallapproaching condition.

References Cited UNITED STATES PATENTS 2,507,367 5/1950 Carbonara et a1.73-178 2,538,303 1/1951 Findley 73-178 2,799,461 7/ 1957 Anderson et a12 44-77 3,128,967 4/1964 Hays 244-77 3,143,319 8/1964 Gorharn et al.24477 MILTON BUCHLER, Primary Examiner.

B. BELKIN, Assistant Examiner.

1. FOR USE IN AN AIRPLANE INSTRUMENT THAT CONTROLS THE AIR SPEED OF ANAIRPLANE DURING LANDING APPROACH, MEANS TO FURNISH A BIAS SIGNAL TOCOMMAND AN INCREASE IN AIR SPEED TO OFFSET THE EFFECT OF NEGATIVE GUSTSON AIR SPEED DURING LANDING APPROACH, SAID MEANS COMPRISING A FIRSTMEANS HAVING AN OUTPUT WHICH IS RESPONSIVE TO GUST ENGENDEREDFLUCTUATIONS IN AIR SPEED, A SECOND MEANS HAVING AN OUTPUT WHICH IS AFUNCTION OF THE DIFFERENCE BETWEEN THE PREVAILING AVERAGE LIFT CONDITIONOF THE AIRPLANE AND A PREDETERMINED DESIRABLE NULL LIFT CONDITION, THEOUTPUTS OF THE FIRST AND SECOND MEANS BEING IN THE SAME NEGATIVE SENSEFOR DECREASING AIR SPEED, A THIRD MEANS FOR COMPARING THE OUTPUTS OF THEFIRST AND SECOND MEANS AND FURNISHING A DIFFERENCE OUTPUT, AND ANINTEGRATOR RESPONSIVE TO AND FOR STORING SUBSTANTIALLY ONLY A NEGATIVEOUTPUT OF THE THIRD MEANS AS SAID BIAS SIGNAL, SUCH NEGATIVE OUTPUTCORRESPONDING TO AN APPROACH TO STALL.